Scientists have used complex statistical analysis to assess one of the most dramatic changes in the history of visible life on Earth. At the end of the Permian, during a mass extinction event there was a dramatic and extensive faunal turnover between brachiopods and bivalves.

One of the biggest crises in Earth’s history was marked by a revolution in the shellfish. Brachiopods, sometimes called “lamp shells”, as some genera superficially resembled Roman lamps, were replaced everywhere ecologically by the bivalves, such as clams, mussels and oysters. This happened as a result of the devastating end-Permian mass extinction which reset the evolution of life 250 million years ago.

Research conducted by palaeontologists based in Wuhan (China) and the University of Bristol, has shed new light on this crucial faunal turnover when ocean ecosystems changed, eventually taking on a more modern, familiar structure that still persists today.

Brachiopods and Bivalves

Life on land and in the sea is rich and forms particular ecosystems. In modern oceans, the seabed is dominated by animals such as bivalves, corals, gastropods, crustaceans, marine worms and fishes. These ecosystems all date back to the Triassic when life slowly recovered from the “Great Dying”. During that crisis, only one in twenty species survived, and there has been long debate about how the new ecosystems were constructed and why some groups survived, and others perished.

Brachiopods were the dominant shelled animals prior to the extinction. However, bivalves thrived afterwards, seemingly better adapting to their new conditions.

Lead author of the study published in “Nature Communications”, Zhen Guo commented:

“A classic case has been the replacement of brachiopods by bivalves. Palaeontologists used to say that the bivalves were better competitors and so beat the brachiopods somehow during this crisis time. There is no doubt that brachiopods were the major group of shelled animals before the extinction, and bivalves took over after.”

Statistical Bayesian Analysis

Co-author Joe Flannery-Sutherland added:

“We wanted to explore the interactions between brachiopods and bivalves through their long history and especially around the Permian-Triassic handover period. So, we decided to use a computational method called Bayesian analysis to calculate rates of origination, extinction, and fossil preservation, as well as testing whether the brachiopods and bivalves interacted with each other. For example, did the rise of bivalves cause the decline of brachiopods?”

The researchers found that in fact both groups shared similar trends in diversification dynamics right through the time of global crisis.

This suggests that these two groups were not really competing or preying on each other. It is more likely that these unrelated groups were responding to similar external drivers such as fluctuations in sea temperature, oxygen levels and acidity.

The bivalves eventually prevailed, and the brachiopods retreated to deeper waters, where they still occur, but in much reduced numbers.

Statistical Analysis to Resolve the Brachiopods and Bivalves Faunal Turnover Issue

Professor Zhong-Qiang Chen (China University of Geosciences, Wuhan) explained that it was very satisfying to see how modern computational techniques helped resolve a long-standing issue in palaeontology.

Professor Zhong-Qiang Chen stated:

“We always thought that the end-Permian mass extinction marked the end of the brachiopods and that was that. But it seems that both brachiopods and bivalves were hit hard by the crisis, and both recovered in the Triassic, but the bivalves could adapt better to high ocean temperatures. So, this gave them the edge, and after the Jurassic, they just rocketed in numbers, and the brachiopods didn’t do much.”

Fossils of over 330,000 brachiopods and bivalves were analysed in the course of this study. The Bristol University supercomputer took weeks to crunch all the numbers. The Bayesian analysis took into account all kinds of uncertainties and aspects of the data to provide an extremely detailed report on the evolutionary changes.

Professor Michael Benton (University of Bristol) concluded:

“The end-Permian mass extinction was the biggest of all time, and it massively reset evolution. In fact the 50 million years after the crisis, the Triassic, marked a revolution in life on land and in the sea. Understanding just how life could come back from near-annihilation and then set the basis for modern ecosystems is one of the big questions in macroevolution. I’m sure we haven’t said the last word here though!”

Everything Dinosaur acknowledges the assistance of a media release from the University of Bristol in the compilation of this article.

The scientific paper: “Bayesian analyses indicate bivalves did not drive the downfall of brachiopods following the Permian-Triassic mass extinction” by Zhen Guo, Joseph T. Flannery-Sutherland, Michael J. Benton, and Zhong-Qiang Chen published in Nature Communications.

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Long necks in proportion to overall body length is known in many tetrapods. Giraffes and sauropods are typical examples. The evolution of a longer neck being linked to feeding strategies. A newly described ancestor of plesiosaurs named Chusaurus xiangensis suggests that neck elongation occurred rapidly in these types of marine reptiles. Lengthy necks, ideal for pursuing fast-moving nektonic prey such as fish and squid developed quickly over a five-million-year period approximately 250 million years ago.

Picture credit: Qi-Ling Liu

Chusaurus xiangensis

Researchers have reported a new species of pachypleurosaurid sauropterygian from southern China. The new species shows key features of its Middle Triassic relatives, but has a relatively short neck, measuring 0.48 of the trunk length, compared to > 0.8 from the Middle Triassic onwards. Comparative phylogenetic analysis shows that neck elongation occurred rapidly in all Triassic eosauropterygian lineages. This evolution was probably driven by feeding pressure in a time of rapid re-establishment of new kinds of marine ecosystems.

The lengthy necks of marine reptiles, used for chasing fast-moving fishes, developed quickly over a five-million-year period around 250 million years ago.

Adding More Vertebrae

The researchers conclude that pachycephalosaurs lengthened their necks mainly by adding new vertebrae.

The findings, published today in BMC Ecology and Evolution, and carried out by scientists in China and the UK, show that pachypleurosaur taxa lengthened their necks mainly by adding new vertebrae. One taxon, Keichousaurus had more than 20 cervical vertebrae, while some Late Cretaceous plesiosaurs such as Elasmosaurus had as many as 72. Its neck was five times the length of its trunk.

Picture credit: Everything Dinosaur

The illustration (above) was inspired by the recently introduced CollectA Age of Dinosaurs Elasmosaurus figure.

To view this range of prehistoric animal figures: CollectA Age of Dinosaurs Prehistoric Life Models.

Pachypleurosaurs Like Chusaurus xiangensis Evolved in the Early Triassic

These reptiles originated in the Early Triassic, four million years after the end-Permian mass extinction that wiped out around 90% of Earth’s species. Ecosystems were undergoing dramatic changes in the aftermath of the extinction event.

The authors of the study, including scientists from the University of Bristol, studied the Chusaurus xiangensis fossils from Hubei Province (China). Its neck had begun to lengthen. However, it was less than half the length of its trunk, compared to later relatives that had a neck length to trunk ratio of greater than 0.8 (80%).

Two Fossil Skeletons to Study

Lead researcher Qi-Ling Liu from the China University of Geosciences (Wuhan), commented:

“We were lucky enough to find two complete skeletons of this new beast. It’s small, less than half a metre long, but this was close to the ancestry of the important group of marine reptiles called Sauropterygia. Our new reptile, Chusaurus, is a pachycephalosaur, one of a group of small marine predators that were very important in the Triassic. I wasn’t sure at first whether it was a pachypleurosaur though because the neck seemed to be too short.”

Co-author Dr Li Tian (China University of Geosciences) added:

“The fossils come from the Nanzhang-Yuan’an Fauna of Hubei. This has been very heavily studied in recent years as one of the oldest assemblages of marine reptiles from the Triassic. We have good quality radiometric dates showing the fauna is dated at 248 million years ago.”

Fellow author Professor Michael Benton of the University of Bristol’s School of Earth Sciences explained:

“The end-Permian mass extinction had been the biggest mass extinction of all time and only one in twenty species survived. The Early Triassic was a time of recovery and marine reptiles evolved very fast at that time, most of them predators on the shrimps, fishes and other sea creatures. They had originated right after the extinction, so we know their rates of change were extremely rapid in the new world after the crisis.”

Not All Vertebrates Evolve in the Same Way

Not all vertebrates evolve in the same way. When it comes to evolving a lengthy neck, giraffes have changed in a different way to pachypleurosaurs. Most mammals have seven neck vertebrae. Giraffes have seven neck bones too. Each one is extremely long, so these herbivores can browse on the tops of trees. Chusaurus had seventeen. Later pachycephalosaurs had twenty-five. Late Cretaceous plesiosaurs such as the huge Elasmosaurus had seventy-two. These long necks with numerous vertebrae are likely to have been extremely flexible. These marine reptiles could whip their necks round and grab a fish, whilst keeping their body steady.

Flamingos also have long necks so they can reach the water to feed. They have extra cervical vertebrae, up to twenty, but each one is also long.

Chusaurus xiangensis – Perfectly Adapted to its Environment

Dr Benjamin Moon, who also collaborated in this study stated:

“Our study shows that pachycephalosaurs doubled the lengths of their necks in five million years, and the rate of increase then slowed down. They had presumably reached some kind of perfect neck length for their mode of life.”

Dr Moon added:

“We think, as small predators, they were probably mainly feeding on shrimps and small fish, so their ability to sneak up on a small shoal, and then hover in the water, darting their head after the fast-swimming prey was a great survival tool. But there might have been additional costs in having a much longer neck, so it stabilised at a length just equal to the length of the trunk.”

Everything Dinosaur acknowledges the assistance of a media release from the University of Bristol in the compilation of this article.

The scientific paper: “Rapid neck elongation in Sauropterygia (Reptilia: Diapsida) revealed by a new basal pachypleurosaur from the Lower Triassic of China” by Qi-Ling Liu, Long Cheng, Thomas L. Stubbs, Benjamin C. Moon, Michael J. Benton, and Li Tian published in BMC Ecology and Evolution.

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Two new abelisaurs have been described from fragmentary fossils from the upper Maastrichtian phosphates of the Ouled Abdoun Basin, in northern Morocco. There were at least three, coeval abelisaurid taxa present in Morocco during the Late Cretaceous. Scientists report a growing body of evidence to suggest that dinosaurs were highly diverse in North Africa prior to the end-Cretaceous mass extinction.

Although the two new abelisaurids have yet to be formally described, the fossils associated with them are thought to represent mature animals and they demonstrate enough autapomorphies for researchers to be confident that they represent new genera.

Writing in the academic journal “Cretaceous Research”, the researchers conclude that these different-sized carnivores co-existed. The break-up of the super-continents led to the establishment of dinosaur dominated regional biotas. The Late Cretaceous dinosaur fauna of North Africa remains relatively unknown in comparison to North America and Asia. The discovery of these fossils indicates that there were several species of predatory dinosaur present in North Africa. The presence of so many predators suggests that dinosaurs were still thriving in Africa immediately prior to the end-Cretaceous mass extinction event.

Picture credit: Andrey Atuchin

Two New Abelisaurs Described from Fragmentary Fossils

The fossils come from the phosphate mines of the Ouled Abdoun Basin. These strata were deposited in a shallow marine environment along the eastern margin of the Atlantic Ocean. One genus, found near the town of Sidi Daoui, is represented by a foot bone (metatarsal) from an abelisaurid about two and a half metres long.

Picture credit: University of Bath

Metatarsal Fossil Shows Unusual Characteristics

The metatarsal shows strong mediolateral compression, a feature present in noasaurids and some early abelisaurids, but absent in most Late Cretaceous abelisaurids. It is distinct from other abelisauroids in the strong constriction and bowing of the shaft in lateral view, and the medial curvature of the bone in anterior view. Bone texture suggests it comes from a mature individual. The small size, gracile proportions and unusual shape of the metatarsal suggest it is not closely related to other latest Cretaceous abelisaurids.

The other fossil specimen, comes from nearby Sidi Chennane. It is a partial right tibia (shin bone) of a theropod estimated to be around five metres in length.

Picture credit: University of Bath

Lead author of the study, Dr Nick Longrich, from the Milner Centre for Evolution at the University of Bath, commented:

“What’s surprising here is that these are marine beds. It’s a shallow, tropical sea full of plesiosaurs, mosasaurs, and sharks. It’s not exactly a place you’d expect to find a lot of dinosaurs. But we’re finding them.”

Revealing a Diverse Late Cretaceous Dinosaur Fauna

Dinosaur fossils from these strata are rare. However, the small number of dinosaur fossils that have been recovered represent five different species – a small hadrosaur named Ajnabia odysseus and a long-necked and as yet unnamed titanosaur. Theropods are represented by the giant abelisaurid Chenanisaurus (C. barbaricus), and the two new abelisaurs.

To read Everything Dinosaur’s blog post about the discovery of Ajnabia: The First Hadrosaurid From Africa.

To read Everything Dinosaur’s article from 2017 about the discovery of Chenanisaurus barbaricus: The Last Dinosaur from Africa.

Everything Dinosaur acknowledges the assistance of a media release from the University of Bath in the compilation of this article.

The scientific paper: “New fossils of Abelisauridae (Dinosauria: Theropoda) from the upper Maastrichtian of Morocco, North Africa” by Nicholas R. Longrich, Erik Isasmendi, Xabier Pereda-Suberbiola and Nour-Eddine Jalil published in Cretaceous Research.

Visit the award-winning Everything Dinosaur website: Everything Dinosaur.